Macromolecules 1992,25, 1801-1808
1801
Cocrystallization and Phase Segregation of Polyethylene Blends. 1. Thermal and Vibrational Spectroscopic Study by Utilizing the Deuteration Technique Kohji Tashiro,*-+ Richard S. Stein, and Shaw L. Hsu Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Received May 16, 1991; Revised Manuscript Received December 4, 1991 ABSTRACT Crystallization behavior has been investigated by DSC and Fourier transform infrared spectroscopic methods for the 50/50wt % blends of deuterated high-density polyethylene (DHDPE) with hydrogeneous PE species having different branching content. A utilization of the fully deuterated sample as one component has made it possible to trace the crystallization behaviors of the Hand D speciesseparately by measuring the CH2 and CD2 vibrational bands as a function of temperature. The blend between DHDPE and linear low-density PE (LLDPE) with a relatively low degree of short-chain branching has been found to exhibit a cocrystallization phenomenon even in the slow cooling process from the melt. For the blend with high-density PE or LLDPE of a high degree of branching, the phase segregation has been observed between the D and H species. In this way the degree of cocrystallizability has been found to correlate with the branching content. The Davydov splitting of the infrared crystalline bands has also been found to be affected by the degree of branching of the H species in the blend.
Introduction Recently much attention has been paid to the polymer blends from scientific and practical points of view. Polyethylene (PE) blends between the samples with different degrees of branching content have also been widely investigated.'-'o In these studies, it is very important and useful to get the information of the structure and crystallizationbehavior of each component separately and a t the molecular level. Unfortunately, however, the two components in the usual P E blends have almost the same chemical structure of carbon and hydrogen atoms. Therefore, it is very difficult to distinguish the behaviors of the components separate1y.'l-l2 Then we tried here to use a deuterated P E sample as one component. This idea comes from such a fact that the infrared bands of CH2 and CD2 groups appear a t different frequency positions. That is to say, by tracing the change in the infrared bands of CH2 and CD2 species, we may clarify the crystallization behavior of the H and D species and their correlation. Another interesting point is a problem of the cocrystallization and phase segregationbetween the H and D species in the P E blend.13J4 (There have been many discussions on the blends between the H and D species such as highdensity and linear low-density P E blends, e t ~ . l - ~For ) example, in the blend between the deuterated HDPE (DHDPE) and hydrogeneous HDPE, the phase segregation was reported to occur when the sample was slowly cooled from the melt,1k1s but such a segregation has been found not to occur in the blend of DHDPE with linear low-density P E (LLDPE) as described here. In this paper we will measure the differential scanning calorimetry and the temperature dependence of the infrared spectra during the heating and cooling processes from the melt in order to clarify the problem of cocrystallization and phase separation in the PE blend system. In a study on blends, detailed information is also needed on the relationship between the structure (crystal structure, lamellar structure, and spherulite structure) and crystallization behavior as seen at different levels of viewpoint from molecular dimension to the higher dit Permanent address: Department of Macromolecular Science, Faculty of Science,Osaka University,Toyonaka, Osaka 560, Japan.
mension. Then we carried out the experiments of wideangle and small-angle X-ray scatterings and small-angle light scattering in parallel with the present infrared study. The results will be reported in a separate paper.20
Experimental Section Samples. In this study the deuterated high-density PE (DHDPE) sample was used as one component, which was purchased from Merck ChemicalsCo., L a . The hydrogeneousPE samples with different degrees of branching were supplied from Euron ChemicalsCo., LM. (For the LLDPE samples the side chain is an ethyl group.) They were rinsed carefully by using n-hexane to remove the process oil (about 1%content). The characterization results of these samples are listed in Table I, in which the molecular weights were evaluated using GPC and the branching content was estimated from the viscosity measurement. The names of the sampleswere abbreviated as HDPE for high-density PE, LLDPE for linear low-densityPE, and LDPE for low-density PE. Theblendswere prepared by dieaolvingthe two equiweighted species(50wt % DHDPE and 50wt % HDPE, which is equivalent to0.875 mol of DHDPE/l mol of HDPE) in boilingp-xylenewith a concentration of about 2 wt % and by rapidly precipitating into methanol at room temperature. The films for the infrared measurements were prepared by pressing the samples on the hot stage. Measurements. The DSC thermograms in both the heating and cooling processeswere measuredusing a Du Pont differential scanning calorimeter type 2000, and the data obtained in the second cycle were used in order to eliminate any thermal history. The heating and cooling rates were 10 OC/min. (Sometimes 2 OC/min, etc., was also used in order to investigate the heating/ cooling rate dependence of the thermograms, but no essential difference was detected.) The temperature dependencesof Fourier transform infrared (FTIR) spectra were measured in both the heating and cooling processeswith a rate of ca. 1OC/min.The used FTIR spectrophotometer was an IBM FTIR type 32 with an instrumental resolution of 2 cm-'. Results and Discussion DSC Measurements. In Figures 1 and 2 are reproduced the DSC thermograms measured for the pure P E samples listed in Table I in the heating and cooling processes, respectively. The melting behavior becomes gradually broad as the branching content is increased from HDPE to LLDPE(3). DHDPE has a melting point about
0024-9297f9212225-1801$03.00/0 0 1992 American Chemical Society
Macromolecules, Vol. 25, No. 6, 1992
1802 Tashiro et al.
DHDPE + HDPE
Table I Characterization of PE Samples Mn 14K 24K 27K 37K 20K 22K
M m
80K 126K 121K 75K 61K 181K
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Temperature/% Figure 4. Comparison in the DSC thermogram between the blend and a mixture of the H and D species in the DHDPE + LLDPE(2)system:(upper)heating and (lower)coolihgprocesees. and D species;i.e., the blocks of the two specieswere packed into the DSC sampling pan. In the thermogram of the DHDPE + HDPE system (Figure 31, apparently one but actually two overlapping peaks are observed in the heating process:they are slightly different in position from those of the original pure samples. In the cooling process, too, the two overlapping peaks are detected for both the blend and mixture although the peak positions are difficult to read out definitely. (It should be noted that the rising-up temperature of the curve is almost coincident with those of the original pure samples.) For the DHDPE LLDPE(2) blend system shown in Figure 4, one sharp peak can be clearly observed in both the heating and cooling
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Macromolecules, Vol. 25, No. 6, 1992
Cocrystallization and Phase Segregation of PE Blends. 1 1803 DHDPE + LLDPEIPI
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